A PILOT-PLANT TO TREAT CHROMIUM-CONTAMINATED GROUNDWATER

The groundwater at a site in California is contaminated with Hexavalen t Chromium. Different treatment options were tested The options tested included: carbon adsorption, ion exchange chemical treatment reverse osmosis, and in-place fixation. Electrochemical treatment was the trea tment of choice. Treatment operations were designed with turn down fle xibility to allow operation at variable flow rates. Metal reduction is the first treatment step after collection of the groundwater and lowe ring of the pH to 3 in an on-line acid tank. Soluble ferrous ions are produced in an off-line electrochemical cell using sacrificial electro des, as represented by the following equation: Fe-o --> Fe+2 + 2e - (1 ) Using a slight excess of the stoichiometric 3.2 pounds of iron per p ound of chromium, the ferrous ions are mixed with the contaminated gro undwater. In the resulting redox reaction Hexavalent Chromium is reduc ed to the insoluble trivalent state and divalent iron is oxidized to i ts insoluble trivalent state, as represented by the following equation : 3Fe(OH)(2) + CrO4-2 + 4H(2)O (2) -->3Fe(OH<INF>3</INF>) + Cr(OH)<INF >3</INF> + 2OH<SUP>-1 </SUP>As stated, a slight excess of ferrous ion, adjusted pH, and sufficient residence time drive the reaction to comp letion. After the reaction is completed the pH is adjusted to a slight ly alkaline state (8) to allow precipitation of the metal ions in a pH adjustment tank. The stream is advanced to a clarifier where a polyel ectrolyte polymer is added to promote coagulation of the reduced metal and consequently, settlement at the bottom of the clarifier Solids ar e drawn from the bottom of the clarifier and allowed to settle in a gr avity thickener followed by a plate and frame filter press. The treate d groundwater is pH adjusted to below 8 and polished with multimedia f ilters and carbon prior to reinjection to the aquifer. The water is co ntinually checked for total dissolved solids and total chromium concen tration. The chrome concentration is continuously kept below 30 PPB; h owever, the total dissolved solids (TDS) oscillate between 1200 to 240 0 PPM, depending on the plant TDS influent concentration which ranges from 400-700 PPM and the chromium influent concentration which ranges between 25 000 PPB and 5000 PPB.(()